The present invention relates to a cold-rolled steel sheet and a galvanized steel sheet, which are excellent in formability, panel shapeability, and dent-resistance required for an outer panel of a motor car, and a method of manufacturing the same.
An excellent formability, a satisfactory shape after a panel formation and a high dent-resistance (resistance to local depression) are required for a steel sheet for an outer panel of a motor car. The panel formability is evaluated by indexes such as yield strength, elongation, and an n-value (work-hardening index) of the steel sheet. Also, the panel shapeability and the dent-resistance are evaluated in many cases by yield strength and the yield strength after the working and the coating-baking treatment. If the yield strength of the steel sheet is weakened, the press formability can be improved. However, the dent-resistance after the panel formation is rendered unsatisfactory. On the other hand, if the yield strength of the steel sheet is increased, the dent-resistance is improved. However, problems are generated in terms of the press formability such as occurrence of wrinkles or cracks. Such being the situation, vigorous researches are being made in an attempt to obtain a steel sheet having a low yield point in the press forming and a high yield strength after the forming and baking as an outer panel for a motor car. As a cold-rolled steel sheet meeting these two contradictory requirements in terms of the yield strength, a bake-hardenable steel sheet, hereinafter referred to as a xe2x80x9cBH steel sheetxe2x80x9d, utilizing a strain aging phenomenon of the carbon atoms within the steel has been developed.
Particularly, known is a method of manufacturing a BH steel sheet having an excellent deep drawability, which is a cold-rolled steel sheet prepared by adding elements capable of forming carbonitrides such as Nb and Ti to a steel having a very low carbon content of about 50 ppm, the addition amount of such an element being not larger than 1 in terms of the atomic ratio of carbon. For example, Japanese Patent Publication (Kokoku) No. 60-46166 teaches that a Nb or Ti added low-carbon steel is annealed at a high temperature close to 900xc2x0 C. for manufacturing the particular BH steel sheet. Also, Japanese Patent Disclosure (Kokai) No. 61-276928 teaches that an extra low carbon BH steel sheet is manufactured by annealing under a temperature region of about 700 to 850xc2x0 C.
The technology disclosed in JP ""166 is certainly advantageous in that the BH properties and an r-value can be improved. However, since the annealing is performed at a high temperature, the rough surface derived from enlargement of the ferrite grains is worried about. In addition, since the steel sheet itself is softened, the yield strength after the press forming and the baking steps is not acceptably high, though high BH properties may be obtained. On the other hand, in the technology disclosed in JP ""928, the annealing temperature is relatively low, compared with that employed in JP ""166, and, thus, is desirable in the required surface properties and the yield strength. However, it is substantially impossible to improve as desired the BH properties and the r-value. It should also be noted that these prior arts are mainly intended to improve the BH properties of a steel sheet in order to allow the steel sheet to exhibit an improved dent-resistance. Therefore, deterioration in the resistance to natural aging, i.e., occurrence of stretcher strain in the press forming, which is derived from generation of a yield point elongation during storage under room temperature, is worried about. Under the circumstances, the BH amount is suppressed at 60 MPa or less in view of the practical use of the steel sheet.
As described above, the cold-rolled sheet manufactured by the conventional method is not sufficiently satisfactory in the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car.
An object of the present invention is to provide a cold-rolled steel sheet and a galvanized steel sheet, which are satisfactory in any of the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car, and a method of manufacturing the same.
(1) The present invention provides a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, the cold-rolled steel sheet meeting the relationship given in formula (2):
1.0xe2x89xa6(Nb %xc3x9712)/(C %xc3x9793)xe2x89xa63.0xe2x80x83xe2x80x83(1)
xe2x80x83exp(xcex5)xc3x97(5.29xc3x97exp(xcex5)xe2x88x924.19)xe2x89xa6"sgr"/"sgr"0.2xe2x89xa6exp(xcex5)xc3x97(5.64xc3x97exp(xcex5)xe2x88x924.49)xe2x80x83xe2x80x83(2)
where 0.002 less than xcex5xe2x89xa60.096, xcex5 represents a true strain, "sgr"0.2 represents a 0.2% proof stress, and a represents a true stress relative to "sgr".
(2) The present invention provides the cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in item (1) above, further comprising 0.0001 to 0.002% by weight of B.
(3) The present invention provides a galvanized steel sheet excellent in formability, panel shapeability and dent-resistance, which is obtained by applying a galvanizing to the cold-rolled steel sheet defined in item (1) or item (2) above.
(4) The present invention provides a method of manufacturing a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in item (1) or item (2) above, comprising the steps of:
preparing a molten steel and continuously casting the steel;
applying a hot-rolling process such that a finish rolling is performed at (Ar3xe2x88x92100)xc2x0 C. or more and the rolled steel sheet is coiled at 500 to 700xc2x0 C.; and
continuously applying a cold-rolling process and an annealing process to the hot-rolled steel sheet.
(5) The present invention provides a method of manufacturing a galvanized steel sheet, the steel sheet being excellent in formability, panel shapeability and dent-resistance, defined in item (3) above, comprising the steps of:
preparing a molten steel and continuously casting the steel;
applying a hot-rolling process such that a finish rolling is performed at (Ar3xe2x88x92100)xc2x0 C. or more and the rolled steel sheet is coiled at 500 to 700xc2x0 C.; and
continuously applying a cold-rolling process and a galvanizing process to the hot-rolled steel sheet.
(6) The present invention provides a cold-rolled steel sheet excellent in the surface shape of a panel and dent-resistance, comprising 0.004 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.1 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.005 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.005% by weight of N, and at least one kind of the element selected from the group consisting of 0.02 to 0.12% by weight of Nb and 0.03 to 0.1% by weight of Ti, the amount of C, Nb, Ti, N and S meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, the cold-rolled steel sheet meeting the relationship given in formula (2):
xe2x80x83xe2x88x920.001xe2x89xa6C %xe2x88x92(12/93)Nb %xe2x88x92(12/48)Ti*xe2x89xa60.001xe2x80x83xe2x80x83(1)
where Ti*=Ti %xe2x88x92(48/14)N %xe2x88x92(48/32)S %, when Ti* is not larger than 0, Ti* is regarded as 0.
exp(xcex5)xc3x97(5.29xc3x97exp(xcex5)xe2x88x924.19)xe2x89xa6"sgr"/"sgr"0.2xe2x89xa6exp(xcex5)xc3x97(5.64xc3x97exp(xcex5)xe2x88x924.49)xe2x80x83xe2x80x83(2)
where 0.002  less than xcex5xe2x89xa60.096, xcex5 represents a true strain, "sgr"0.2 represents a 0.2% proof stress, and "sgr" represents a true stress relative to xcex5.
(7) The present invention provides a cold-rolled steel sheet excellent in the surface shape of a panel and dent-resistance defined in item (6) above, further comprising 0.0001 to 0.002% by weight of B.
(8) The present invention provides a galvanized steel sheet, the steel sheet being excellent in the surface shape of a panel and dent-resistance and prepared by applying a galvanizing to the cold-rolled steel sheet defined in item (6) or item (7) above.
(9) The present invention provides a method of manufacturing a cold-rolled steel sheet excellent in the surface shape of a panel and dent-resistance and defined in item (6) or item (7) above, comprising the steps of:
applying a hot-rolling process after preparation of a molten steel and continuous casting of the steel such that a finish rolling is performed at (Ar3xe2x88x92100)xc2x0 C. or more and the rolled steel sheet is coiled at 500 to 700xc2x0 C.; and
continuously applying a cold-rolling process and an annealing process to the hot-rolled steel sheet.
(10) The present invention provides a method of manufacturing a galvanized steel sheet, the steel sheet being excellent in the surface shape of a panel and dent-resistance and defined in item (8) above, comprising the steps of:
applying a hot-rolling process after preparation of an ingot steel and continuous casting of the ingot steel such that a finish rolling is performed at (Ar3xe2x88x92100)xc2x0 C. or more and the rolled steel sheet is coiled up at 500 to 700xc2x0 C.; and
continuously applying a cold-rolling treatment and a galvanizing treatment to the hot-rolled steel band.